Identification of the TW Hydrae association member 2M1235-39: a tertiary component of the HR 4796 system

Aims. We seek to determine whether the late-type star 2MASS J12354893−3950245 (2M1235−39) is a member of the TW Hya Association (TWA), a hypothesis suggested by its association with a bright X-ray source detected serendipitously by ROSAT and XMM-Newton and its (∼ 3 ' ) proximity to the well-studied (A+M binary) system HR 4796. Methods. We used optical spectroscopy to establish the Li and Hline strengths of 2M1235−39, and determined its proper motion via optical imaging. We also considered its X-ray and near-IR fluxes relative to the M star HR 4796B. Results. The optical spectrum of 2M1235−39 displays strong Li absorption and Hemission (equivalent widths of 630 mA and−6.7 A, respectively). Comparison of the spectrum with that of a nearby field star, along with the DENIS catalog I JK magnitudes, indicates the spectral type of 2M1235−39 is M4.5. We measure a proper motion for 2M1235−39 that agrees, within the errors, with that of HR 4796. Conclusions. The Li absorption and Hemission line strengths of 2M1235−39, its near-IR and X-ray fluxes, and its proper motion all indicate that 2M1235−39 is a TWA member. Most likely this star is a wide (13,500 AU) separation, low-mass, tertiary component of the HR 4796 system.